PSI - Issue 60

S.K. Chandra et al. / Procedia Structural Integrity 60 (2024) 203–213 Author name / StructuralIntegrity Procedia 00 (2019) 000 – 000

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geometries used for CTOA measurements according to ASTM E 2472 (ASTM E2472, 2012) has been adopted to choose the dimensions for DENT and SENT specimens while taking account of the following two factors: 1. The ligament length should be such that ≫ 4 so to cover the expected initial transients in , and 2. Free length between grips should be large enough that plastic zone does not spread to the gripping section of the specimens. Fatigue pre-cracking of SENT and DENT specimens were carried out under load-control mode with a sinusoidal waveform by keeping the maximum allowable fatigue load to 0.3 times of the limit load and a load ratio to 0.1. The DENT specimens were flipped (horizontally and vertically) several times to maintain the symmetry of the growing pre-crack; maintaining the symmetry of the pre-crack was not an issue with SENT specimens because of a single crack starter notch. The extent of pre-cracking was optically measured on both surfaces, and the average values for crack growth was recorded. The length of pre-crack was measured using a stereo microscope (Leica) with magnification that resulted in a proper identification of pre-crack (usually 10X and 20X). Periodic load shedding after about every 10,000 cycles was adopted in order to maintain the intended maximum load and load ratio. A typical pre-cracking process required about 80,000 cycles for the intended crack growth.

Fig. 1. Schematic diagram of (a) SENT and (b) DENT specimen. In the present campaign, the interest was in generating both − ∆ and 5 −∆ plots. Since for  5 measurements the recommended  5 gage was not available, an alternative method using microhardness indentation marks placed at distance ±2.5 mm from crack tip parallel to the loading axis (A-A1 line in Fig. 2) was used for optical measurement of CTOA ( ) on the test specimen. Actually, as shown in Fig. 2, two more pair of points placed 0.25 mm apart, at the same distance from the crack path were added. The intention was: in the deformed specimen, on each side of the crack path, the placing of three points would help in unequivocally identifying the gage points for optical measurement.